The present exemplary embodiment relates to an apparatus and method for the improved fabrication of small particles, which may also be described as beads or balls. The embodiment finds specialized application in the fabrication of bichromal particles, for a twisting ball display and more specifically to improved control of system parameters resulting in improved uniformity of the final product.
A common procedure for making small particles is known as bead spinning, which involves flowing a liquid onto a spinning disk. The liquid is carried to the outside perimeter of the disk by centrifugal force. Droplets of liquid fly off the edge of the disk and, by drying and/or cooling, solidify. The droplets are collected as solid particles or used to form a film on a surface.
Bichromal balls have hemispheres of contrasting colors and find particular use in “electric paper” displays. Electric paper displays are described in a number of patents including U.S. Pat. Nos. 4,126,854 and 4,143,103 assigned to the present assignee.
One method for making bichromal balls is described in U.S. Pat. No. 5,262,098, which is assigned to the present assignee and is herein fully incorporated by reference. This patent describes an additional step to the traditional bead spinning procedure described above. One liquid is flowed on the top side, of the spinning disk and another liquid is flowed on the bottom side of the same disk. As the disk spins, both liquids are drawn to the outside perimeter of the disk. As the liquids reach the edge of the disk, the liquid on the top of the disk and the liquid on the bottom of the disk merge into a droplet composed of a top side liquid hemisphere and a bottom side liquid hemisphere. The composite droplets then fly off the edge of the disk and solidify prior to landing. When the top side liquid is a contrasting color from the bottom side liquid, a bead with hemispheres of contrasting colors results.
Applications using these particles generally require the particles be very small. The patents cited above refer to bichromal balls in the range of 5 microns to 500 microns. Variations of size and shape of the final product particles may result in nonfunctionality. Due to the ease of nonconformity and the relatively small size of each particle, a wasteful and time-consuming sieving process must be undertaken to narrow the size distribution within that particular batch to an acceptable range.
In an attempt to control the size ranges of the particles and minimize or eliminate the wasteful sieving practice, or improve the uniformity of the film, droplet break-off was analyzed. In both the traditional bead spinning method and the bichromal ball spinning method, the droplet break-off at the edge of the disk is controlled by many factors. These factors include: surface tension, viscosity, the shape of the disk edge, air currents and disk rotations per minute (rpm). Small variations in any of these factors can often result in unacceptable variation in drop size. In an effort to minimize the variations, efforts are made to control the above variables. While some improvement has been seen, the waste from the process is still at undesirable levels.